1. Field of the Invention
The present invention relates to a sheet folding device that folds a sheet or a sheet bundle, a sheet processing apparatus including the sheet folding device, and an image forming system including an image forming apparatus such as, a copying machine, a printer, a facsimile machine, or other similar image forming apparatuses, and the sheet processing apparatus.
2. Discussion of the Background
Conventional sheet processing devices have been widely used for stapling, punching, and folding sheets on which images are formed in the image forming apparatus. These conventional devices are generally disposed on a downstream side of an image forming apparatus such as, a copying machine, a printer, a facsimile machine, or other similar image forming apparatuses, in a sheet conveying direction. A more recent conventional sheet processing apparatus has multiple functions including center staple processing in which staple pins are stapled at at least two portions of a central portion of a sheet bundle, in addition to end-staple processing in which a staple pin is stapled at one portion near a comer portion of a sheet bundle. Generally, when performing center-staple processing, conventional center-folding processing is also performed in which a sheet bundle is folded at its stapled central portion and is simply bookbound.
The above-described conventional sheet processing apparatus uses a pair of folding rollers as folding members. A sheet bundle is folded in two by pinching the stapled central portion of the sheet bundle at a nip part between the folding rollers to bind the sheet bundle as a booklet. In this simple book-binding operation, it is desirable to neatly align the edge of the booklet. However, if the sheet bundle is skewed before being folded at the nip part of the folding rollers, the quality of the booklet is degraded.
In another conventional sheet processing apparatus described in Published Japanese patent application No. 2001-146363, a leading edge portion of a sheet bundle is abut against a stopper, and a drive device drives the stopper to start reciprocating motions in a sheet bundle conveying direction to align the edge of the sheet bundle before performing folding processing on the sheet bundle. However, in this conventional sheet processing apparatus, time and a complicated mechanism may be necessary for performing the reciprocating motions of the stopper.
Therefore, as discovered by the present inventor, it is desirable to provide a sheet processing apparatus including a sheet folding device in which an edge of a sheet or a sheet bundle can be simply aligned and a skew of a sheet or a sheet bundle can be corrected before performing folding processing on the sheet or the sheet bundle.